An example of conventional radar devices, which is disclosed in U.S. Pat. No. 7,158,217B2 corresponding to Japanese Unexamined Patent Publication No. 2004-177350, is designed to emit laser beams toward the front of a vehicle in which it is installed beforehand. The radar device is also designed to receive beams reflected from an object located in front of the vehicle and detect the location of the object based on the received beams.
Such a radar device is commonly used for alarm systems installed in a vehicle and cruise control systems installed therein. The alarm systems are operative to detect a distance between the vehicle and another one vehicle in front thereof based on the location of another one vehicle detected by the radar device and to give an alarm when the detected distance is shorter than a predetermined threshold distance. The cruise control systems are operative to automatically control the speed of the vehicle so as to maintain a desired interval while following traffic.
Specifically, a radar device disclosed in the US patent Publication and installed in a vehicle uses a rotatably driven polygon mirror to change the direction of a laser beam emitted by a laser diode, thus irradiating a plurality of laser beams over an angular scanning field. The angular scanning field is defined beforehand in the width and height of the vehicle.
When a number of reflected light beams have been sent from an object, which is located in the angular scanning field, to the radar device based on a corresponding number of the transmitted laser beams, the number of reflected light beams are received through a light receiving lens of the radar device to be detected by a photodetector thereof. The photodetector converts the intensity of each of the individual detected light beams into a voltage signal with a level equivalent to the intensity thereof.
Some of the voltage signals corresponding to some of the transmitted laser beams adjacent to each other are integrated so that an amplified voltage signal based on corresponding some of the reflected light beams from the object is obtained. This improves the sensitivity of detection of the object.